In process automation technology, field devices are often applied that serve for registering and/or influencing process variables. For registering process variables, sensors, such as, for example, fill level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH, redox potential measuring devices, conductivity measuring devices, etc., serve to register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, and conductivity. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a pipeline section, or the fill level in a container, can be changed. In principle, all devices are referred to as field devices which are applied close to the process and which deliver, or process, information relevant to the process. Besides the above named sensors and actuators, such units are also generally referred to as field devices, which are directly connected to a fieldbus and which serve for communication with superordinated units, examples of these kinds of field devices including remote I/Os, gateways, linking devices, and wireless adapters. A large number of such field devices are produced and sold by the firm, Endress+Hauser.
In modern industrial plants, field devices are, as a rule, connected with superordinated units via bus systems (Profibus®, Foundation® Fieldbus, HART®, etc.). Normally, superordinated units are control systems or control units, such as, for example, PLCs (programmable logic controllers). The superordinated units serve, among other things, for process control, process visualizing, process monitoring, as well as for field device start-up. The registered, measured values of the field devices, especially values from sensors, are transmitted via the particular bus system to one, or, in given cases, to a number of, superordinated unit(s). In addition, data transmission from the superordinated unit via the bus system to the field devices is also required, especially for configuring and parametering field devices, as well as for actuator operation.
Besides hardwired data transmission between field devices and superordinated unit, there is also the opportunity for wireless data transmission. Especially in the bus systems, Profibus®, Foundation® Fieldbus and HART®, wireless data transmission via radio is provided for. Additionally, radio or wireless networks for sensors according to the standard, IEEE 802.15.4, are specified in greater detail. For implementing wireless data transmission, newer field devices, especially sensors and actuators, are, in part, embodied as radio field devices. These have, as a rule, a radio unit and an electrical current source as integral components. In such case, the radio unit and the electrical current source can be provided in the field device itself or in a radio module permanently connected to the field device. By the electrical current source, a self-sufficient, or autarkic, energy supply of the field device is enabled.
Along with that, the opportunity exists to adapt field devices, especially sensors and actuators, lacking radio units by connecting a wireless adapter to the field device. The wireless adapter has a radio unit, so that the field device becomes a radio field device. Such a wireless adapter is described, for example, in the publication WO 2005/103851 A1. The wireless adapter is, as a rule, releasably connected to a fieldbus communication interface of the field device (especially, an interface of a sensor or actuator). Via the fieldbus communication interface, the field device (especially a sensor or actuator) can transmit the data to be transmitted via the bus system to the wireless adapter, which then transmits these via radio to the target location. Conversely, the wireless adapter can receive data via radio and forward such via the fieldbus communication interface to the field device (especially a sensor or actuator). Supply of the field device (especially the sensor or actuator) with electrical power occurs, as a rule, via an electrical current source of the wireless adapter.
In the case of such radio field devices and wireless adapters, the communication (for example with a superordinated unit) is conducted, as a rule, via the wireless interface of the radio field device or the wireless adapter. Additionally, such radio field devices or wireless adapters include, as a rule, a hardwire communication interface. For example in the HART® standard, it is provided that radio field devices must have, besides a wireless interface, also a hardwire communication interface. Via such a hardwire communication interface, for example, an on-site configuring of the radio field device or the wireless adapter is possible via a service unit, such as, for example, a handheld communicator, connected to the hardwire communication interface. Additionally, the hardwire communication interface can be embodied as a fieldbus communication interface, so that communication is conducted, by way of such, over a bus system, such as, for example, one of the standardized bus systems, Profibus®, Foundation® Fieldbus or HART®. Via such a fieldbus communication interface, the radio field device or the wireless adapter can be connected also to a correspondingly hardwired fieldbus.
The electrical current source of a wireless adapter or of a radio field device is based, for example, on a primary battery, a fuel cell, and/or rechargeable battery, etc., provided in the wireless adapter or in the radio field device. Especially when a field device is embodied as a radio field device or as wireless adapter and is supplied with electrical power via a self-sufficient, or autarkic, electrical current source, or when a field device (especially a sensor or an actuator) supplies a connected wireless adapter with electrical power from the electrical current source, the power consumption of the relevant field device should be minimized. In this way, the lifetime of the electrical current source can be lengthened and, accordingly, the amount of maintenance work to be performed can be reduced. Also in the case of conventional field devices, which are connected to a fieldbus via a hardwire communication interface (fieldbus communication interface), a reduction of energy consumption is desired on cost and environmental grounds.
In times when a field device does not have to do anything, especially in the periods of time between measured value queries, configuration procedures and/or commands for performing a particular action, the relevant field device is, as a rule, placed in a standby mode. Especially, the above described wireless radio field devices and wireless adapters have very low power consumption in standby mode. They require, as a rule, only a few μA, especially less than 10 μA, of electrical current supply. Associated with the above described, hardwire communication interface in the device architecture is a functional unit, by which the sending and/or receiving of digital signals is performed via the hardwire communication interface. The power consumption of such a functional unit is clearly higher than the power consumption of a wireless radio field device or wireless adapter in standby mode. If the functional unit is formed, for example, by an ASIC (application specific integrated circuit), such requires, as a rule, an electrical current supply of more than 100 μA.